How Geysers Form

Introduction

The question, how does a geyser
form, can not be answered quickly. Geysers are temporary features
geologically. The duration or " life span" of a geyser is at the most
several thousand years. Geysers are usually associated with volcanic areas. Their
formation requires the combination of 3 specific geologic conditions that are usually
found in volcanic terrain: 1- intense heat, 2-
Water,
3-a plumbing system. The fact that they need heat
much higher than normally found near the earth's surface is the reason they are usually
associated with volcanoes or volcanic areas. Click below to find out more about the
conditions necessary for geyser formation.

Index

Superheated water is
the needed in order for geysers to form. The
heat comes from a buried magma chamber in an volcanic setting. Water is
more easily obtained than the heat. The water comes from the groundwater
system in the geyser basin. It starts as precipitation and enters the ground
water system by percolating down through porous sand and gravel as well as
cracks and fissures in the earth. The surface water recharges the water that
circulates in the subsurface and is eventually erupted from a geyser or
flows out of hot springs. The water does not need to necessarily be present
in great volumes on the surface. Geyser do occur in desert environments. In
fact some the geysers in Chile
occur in one of the the driest valleys in the world. What is important is
that water is available, and that it can circulate deep into the earth's
surface where it can approach the heat source. It is estimated that it takes
several hundred years for the water in Yellowstone to fall as rain or snow,
enter the groundwater system, circulate deep down near the heat source and
then rise to near the surface where some of it is erupted from geysers.
Surface conditions such as droughts or extremely wet periods may not effect
geyser eruptions or hot spring flow for hundreds of years. Cooler water in
the near surface also mixes with the rising superheated water and may play a
role in a geyser's eruption and timing.

Heat Source

Introduction

The
heat needed for geyser formation comes from liquid rock or magma when it is near the
surface of the earth. That is why geysers and volcanoes are found in the same area.
When the magma reaches the surface, a volcano is formed. Geysers, though rare, are found
in the same geologic settings where volcanic heat sources are available. These settings
are listed below:

In
Divergent Margins, where rifting is occurring, the plates are moving away from each other.
In the oceans, the splitting of the crust or rifting allows magma to reach the surface.
Molten rock spills to the surface along these fractures and new oceanic crust is formed.
Sometimes continental crust is located at the plate boundaries where the plates are or are
starting to move away from each other. On the continental crust when rifting occurs large
cracks known as rift valleys form. As the heat from the mantle rises along these big
cracks, magma forms and rises to the surface which, in turns, results in volcanic
activity. Geysers are found along divergent margins or incipient rift basins where
continental crust is involved. Examples of geysers in this setting are found in
Iceland,
Nevada, and
Kenya. For more information on divergent margins and rifting see
the web pages listed above but make sure you check out this great site:

Hot Spots/
mantle plumes

While
most volcanic activity and related geyser development occur along the plate boundaries
described above, sometimes volcanic activity can be found far from these boundaries.
Yellowstone and the Hawaiian Islands are examples of these types of areas. It is theorized
that these areas are due to hot spots or localized thermal convection in the mantle of the
earth called plumes. The hot spot is stationary and the plates move over them. Much
controversy still is found among geologist about hot spots. Much remains to
be learned about the
mechanism that causes plumes to form. For a thorough description see:

In order for the heated water
to form a geyser a plumbing system is needed. This includes a reservoir to hold the water
while it is heated. The geyser plumbing system is not well understood but we do know that
it is probably formed by a combination of spots of porous rock, in the case of
Yellowstone, gravel deposited by a glacier, and cracks and fissures in the earth. Geysers
often are aligned along faults, the cracks in the earth formed by earthquakes. The
plumbing system is made up of a system of fractures, fissures, porous spaces
and sometimes cavities. Constrictions in this
system are important to the building up of pressure before an eruption. For pictures of
plumbing systems and more info try this site:http://www.umich.edu/~gs265/geysers.html

Silica Seal

If the 3 factors, heat, water and
a plumbing system, are present then flowing hot springs are possible.
Hot springs and fumaroles are very common in volcanic areas, in fact, they are
found in almost every volcanic area on earth. Geysers, however, are found in a small
fraction of the volcanic areas. The factor missing in most areas is strong
rock formations surrounding the thermal areas. Silica linings or
seals on the plumbing system seem to be important to maintain the integrity
of a plumbing system that is subject to the intense pressures caused
by geyser activity. In Yellowstone's geysers basins, silica cements much
of the glacial gravel near the surface and is deposited in white sheets across the
surface. The silica forms a rock called sinter or geyserite. The source of silica is
believed to be rhyolite, a light colored, silica rich volcanic rock. Hot ground waters
dissolved silica from the rhyolite and related rocks and carry it in solution. As the
silica rich water moves near and on the surface the silica comes out of solution and is
deposited as sinter. The sinter coatings on the plumbing system is able to hold back the
pressure and provide a seal that allows the geysers to build enough pressure to erupt.
Without the silica seal, a thermal feature will just be a hot spring and not a geyser.
Most experts agree that the presence of the volcanic rock rhyolite is the key to
the amount and quality of silica needed for geyser development. Large deposits of rhyolite
are rare on or near the earth's surface. One of the notable exceptions are the rhyolites
of the Yellowstone Plateau.

This
photo show silica (sinter) formations that were once found around
Sapphire pool in the Upper Geyser Basin. The silica "biscuits" were
destroyed by large eruptions. Photo by Haynes from the Gregory Jones
Collection.